1. Field of the Invention
The present invention is related to a method for creating a tool travel path on the basis of the size of the tool to be used to perform work on a workpiece by means of a computer numerical control device (hereinafter, called an "NC device").
2. Description of the Prior Art
When a workpiece is worked, for example cut, by an NC device, the path along which the tool is directed to travel must be changed according to the size of the tool.
FIG. 1 is a diagram showing the route of a tool center point corrected according to the diameter of the tool.
Referring to FIG. 1, reference numeral 1 denotes a tool used in the NC device, and reference numeral 4 denotes the workpiece to be worked by the tool 1.
In order to work the workpiece 4 with an NC device, for example, an NC milling machine, the machine operator must initially make an NC program for the tool by using the dimension of the workpiece itself to be worked, irrespective of the tool diameter. Next, the operator may preferably select a tool diameter correcting function, so that the center point of the tool 1 may travel along the desired path, shown by the dotted line, to work the workpiece in the shape denoted by the solid line in FIG. 1.
An NC program generally includes a plurality of unit commands (hereinafter, called a block) which the NC device will execute, for example, a rapid movement of the tool, the cutting of the workpiece, a tool path correction, etc. Each block is commonly separated from other blocks by a semicolon (;).
Additionally, there is a code system called a G-Code (Preparation Function) which is a language for plotting the NC program. Among the various G-Codes, the following are related to this disclosure;
G00: a command for rapidly moving the tool to a designated position; PA1 G01: a command for making the tool move rectilinearly at a specified gradient or in parallel with a specified axis; PA1 G17: a command for selecting a X-Y plane; PA1 G40: a command for canceling a tool path correction command previously given in the NC program; PA1 G41: a command for correcting the tool path in order to direct the tool to travel along the left side of the working surface of the workpiece with respect to the travelling direction (hereinafter, called a "left side correction"); PA1 G42: a command for correcting the tool path in order to direct the tool to travel along the right side of the working surface of the workpiece with respect to the travelling direction (hereinafter, called a "right side correction"); and, PA1 G90: a command for selecting an absolute coordinate system.
FIG. 2 is a diagram showing the travel path (dashed line) of a tool center point according to a left side correction, and FIG. 3 is a diagram showing the travel path of a tool center point according to the right side correction.
When either of the two tool travel path corrections described above is included in the NC program, the creation of the travel path to be corrected is executed simply by adding the radius of the tool to the programmed path.
Accordingly, a problem arises in that the tool cannot be exactly positioned at a desired work starting point, thereby causing the risk of under-cutting or over-cutting the workpiece. Such a problem becomes especially serious when the interior angle between the line segment from the starting point of correction to the work starting point and the line segment from the work starting point to the work ending point is acute.
On the other hand, a typical prior art for a tool diameter correcting method for a numerical controller is disclosed in Japanese patent laid-open publication No. 8002 (1991).
In particular, tool diameter correcting method, which prevents the correction from generating a deviation value after a specified code instruction is executed and which does not require a change in the instruction set value, is disclosed in the Japanese publication. However, a technique for solving the problem described above is not disclosed in the Japanese Publication.